For ions we must adjust the number of electrons available, A

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The Octet Rule Examples

• For ions we must adjust the number of electrons
  available, A
• Add one e- to A for each negative charge
• Subtract one e- from A for each positive charge

• NH4
• BF4

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Example CO32-
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Resonance

• There are three possible structures for CO32-
• The double bond can be placed in one of three
  places

• These are called equivalent resonance structures
• The real structure of the CO32- anion is an
  average of these three resonance structures

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Resonance

• There are no single or double bonds in CO32-
• All three bonds are equivalent
• They are intermediate between the single and
  double bond

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Resonance Other Examples

• SO3

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Resonance Other Examples

• NO3

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Resonance Other Examples

• SO42

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Exceptions to the Octet Rule

• In those cases where the octet rule does not
  apply, the substituents attached to the central
  atom nearly always attain noble gas
  configurations
• The central atom does not have a noble gas
  configuration but may have fewer than 8 or more
  than 8 electrons

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Examples

• BBr3

• AsF5

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Assignments Reminders

• Go through the recent lecture notes
• Read Chapter 7 completely, except for Sections
  7-7 7-8
• Homework 4 due by Oct. 16 _at_ 3 p.m.
• Review Session _at_ 515 p.m. on Sunday

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CHAPTER 8

• Molecular Structure Covalent Bonding Theories

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Stereochemistry

• The study of the three-dimensional shapes of
  molecules
• With the knowledge acquired so far we will be
  able to predict the shapes of molecules and ions
• Our instrument Valence Shell Electron Pair
  Repulsion theory (VSEPR theory R. J. Gillespie)

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VSEPR Theory

• In any molecule or ion there are regions of high
  electron density
• Bonds (shared electron pairs)
• Lone pairs (unshared electrons)
• Due to electron-electron repulsion, these regions
  are arranged as far apart as possible
• Such arrangement results in the minimum energy
  for the system

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BeCl2
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BBr3
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CH4
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PCl5
18
SF6
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Five Basic Geometries
Linear
Trigonal
Tetrahedral
Octahedral
Trigonal bipyramidal
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SiF4
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NH3
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Electronic Geometry and Molecular Geometry

• Electronic geometry
• Distribution of regions of high electron density
  around the central atom
• Molecular geometry
• Arrangement of atoms around the central atom

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H2O